Distributed Antenna Systems (DAS) are commonly used for building indoor coverage of wireless systems. DAS is a network of spatially separated antennas or radiating cables connected to a common source. The antennas or radiating cables are distributed throughout the coverage area and are used for transmitting and receiving signals to/from user equipment. The antennas connected to the common source form a logical cell. The spatial distribution of the antennas provides for well defined coverage with good flexibility using a low output power compared to a single antenna scenario.
Since DAS is defining a logical cell, no conventional handover is needed when user equipment move in the area covered by the antenna system. An evolution of DAS is to introduce an intelligent antenna switching, confining the communication to just one or a few serving antennas in the close vicinity of the user equipment (instead of distributing the signals to/from all antennas). This technique is often referred to as a hub solution or RF switching. A significant interference and power reduction is provided when RF switching is applied With RF switching, it is still possible to avoid handover in the coverage area of the antenna system since the areas covered by means of RF switching would still be confined to one logical cell. In practice, common control channels are broadcasted from all antennas of the logical cell while dedicated and shared channels use RF switching. The serving antenna will hence be changed as the user equipment moves. A DAS with RF switching is hence similar to a macro cell adaptive antenna that switches or steers a beam to the communicating user equipment. Typically the tracking of user equipment could be based on an RF quality measure such as C/I measured at every antenna in uplink. Also, the document ERICSSON “A discussion on some technology components for LTE-advanced” 3GPP Draft R1-082024 Kansas City USA; 20080505 describes an LTE advanced system.
In WCDMA systems a pilot channel is a downlink channel typically having constant power and of a known bit sequence. The pilot channel (i.e. a predefined known signal at the receiver) is used for radio propagation channel estimation. The dedicated information transmitted to a specific receiver is in one example sent via a data transmission channel generated coherently in parallel with the pilot channel utilizing the same serving antenna(s). Accordingly, the data transmission channel experiences the same radio propagation environment as the pilot channel. The pilot channel acts as a phase reference for the data transmission channel. The estimated radio propagation channel can be used coherently at the receiver for detection of the dedicated data transmission. In WCDMA, a Common Pilot Channel (P-CPICH) and some other common control channels would typically be transmitted from all antennas to form and define a logical cell. Optionally, a Secondary Common Pilot Channel (S-CPICH) can be used either in the complete cell defined by the P-CPICH or in sub-areas of the cell as phase reference. In an RF switching indoor system context, an optional S-CPICH can be pre-defined to transmit from only one antenna (or a subset of antennas) of the logical cell and all or selected parts of the logical cell antennas can be assigned different S-CPICH which optionally can be used as phase reference instead of the P-CPICH. The power assignment to the S-CPICH is typically designed statically to support a user at the border of the designed coverage (i.e. a worst case assignment). Even if a user is close the antenna point, with very good quality, there may also be a user at the border prohibiting any potential power and interference savings via power control of the S-CPICH. When the primary CPICH is not used for a user for the purpose of providing a phase reference, the Radio Network Controller (RNC) is arranged to inform the user equipment accordingly.
When the user equipment is moving within the logical cell, it is assigned a new secondary CPICH if it is found that a new antenna is better for serving the user equipment. The RNC is arranged to determine when a new antenna is better suited to serve the user. The RNC is then arranged to assign the user equipment the new S-CPICH (assigned to the new antenna). Therefore, the S-CPICH re-configuration procedure is somewhat similar to performing handover between cells.
There is a latency involved in this operation (signalling between RNC and user equipment). It is desired to minimize the latency since it will degrade user experience. Quality degradation will occur if the wrong S-CPICH is used as phase reference. This problem can be especially severe in high mobility cases where a user quickly moves past a number of antennas. This may for example occur when the user is located in an elevator.